Full scale study of the wash of high speed craft

Abstract

High speed crafts are increasingly replacing conventional low speed crafts. With this increase has come recognition of problems due to speed increase and wave wash associated with it. Pressure is increasing on regulatory bodies to act on the wash induced impacts on the safety of people, craft, ecology and environment. Waves created by high-speed crafts are often significantly high in the far field and the size of these waves increases when the water depth reduces from a very deep to a deep one and then to a shallow region. The effects become magnified when the width of the waterway also gets restricted. So the operation of such crafts needs to be controlled accordingly to avoid damage to environment and risk to persons and other crafts operating in the region. These responsibilities are vested with designer, builder, operator and owner of the craft and also with the port authorities. Therefore it would be useful to estimate the size of the waves created by a vessel before it is put to operation in a region. These estimations should eventually lead to the route optimisation for the existing crafts in their respective operating port, by keeping the wave wash within acceptable limits, and to the evolution of new generation of high-speed crafts with low wave wash. High speed craft generated waves generally comprise of long and short period waves. The height of the long period waves increases when the depth of water decreases and at the same time the wavelength decreases resulting in the increase of wave steepness and this may lead to the breaking of the wave. The form and nature of the breaking of a wave will vary with the slope of the coast and seabed. The level of water will influence the position of the break point and the break point will move closer to the coastline at high water. As longer waves are faster they reach the coast earlier. Short-period waves subside faster than long-period waves and hence longer waves are more observed at greater distance from the vessel track. Interference effects of wave systems generated by different vessels, mainly when they operate in the close proximity, are again of concern to a naval architect. Sailing of vessels through shallow water is more complex than in deep water. In shallow water region the pattern of the wave generated by the vessel is very much dependent on the water-depth based Froude number than on the vessel-length based Froude number. A vessel generates the highest waves when the depth based Fn is about 1.0, the speed of the vessel at this condition being termed as critical speed, which should be avoided. Carrying out experimental studies using vessel models at scaled down water depth can perform the estimation of far field waves. But the tank width limitations of the available hydrodynamic test facilities and the cost and time involved in it may put constraint on the above mode of estimation. As another option the waves can be estimated theoretically. Since the high-speed crafts considered here are slender in form, the linear wave theory can be applied.